1. Field of the Invention
This invention relates to a process for producing a zeolite possessing an MTT-type framework structure, specifically ZSM-23, by employing microwave irradiation, the zeolite produced by the process, membranes and coatings containing this zeolite, and zeolite catalysts containing this zeolite.
2. Discussion of the Related Art
Zeolitic materials, both natural and synthetic, are known to have catalytic properties for various reactions. Certain zeolitic materials are ordered, porous crystalline metallosilicates having a definite crystalline structure as determined by X-ray diffraction. Within these ordered, porous structures there are a number of smaller cavities which can be interconnected by a number of even smaller channels or pores.
Under ideal circumstances the cavities and pores of zeolitic materials are uniform in size. Because the dimensions of these pores allow for the adsorption the molecules of certain dimensions while rejecting other molecules of larger dimensions, zeolitic materials have come to be known as “molecular sieves” and are utilized to exploit this phenomenon of selective adsorption.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates have a rigid three-dimensional framework of SiO4 and AlO4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms such that the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electro valence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation.
This cross-linked framework can be expressed by the relationship of aluminum to the cations, wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K, Cs or Li, is equal to unity. One type of cation may be exchanged entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By using such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation.
Earlier techniques have resulted in the formation of a great variety of synthetic porous, crystalline metallosilicate zeolites. The zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite L (U.S. Pat. No. 3,130,006), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite beta, (U.S. Pat. No. 3,308,069), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-4 (Great Britain Pat. No. 1,117,568), zeolite ZSM-5 (U.S. Pat. No. 3,702,886, now U.S. Pat. No. Re. 29,948), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-22, zeolite, ZSM-23 (U.S. Pat. No. 4,076,842), zeolite ZSM-34, zeolite ZSM-35 (U.S. Pat. No. 4,016,245), zeolite ZSM-39 (U.S. Pat. No. 4,259,306), zeolites ZSM-21 and ZSM-38 (U.S. Pat. No. 4,046,859), ZSM-48 (U.S. Pat. No. 4,375,573), ZSM-51 (U.S. Pat. No. 4,568,654), zeolite EU-1 (European Patent Application 0042 226), zeolite EU-2 (UK Patent Application No. GB 2077709 A), zeolite EU-4 (European Patent Application No. 0 063 436), and zeolites NU-6(1) and NU-6(2) (U.S. Pat. No. 4,397,825), merely to name a few.
Zeolites containing a framework element other than, or in addition to, aluminum, e.g., boron, iron, titanium, zirconium, germanium, gallium, etc., are known from, by example, U.S. Pat. Nos. 3,328,119; 3,329,480; 3,329,481; 4,414,423 and 4,417,088.
The SiO2/Al2O3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO2/Al2O3 ratios of from 2 to 3, and zeolite Y can be synthesized with SiO2/Al2O3 ratios from 3 to about 6. In some zeolites, the upper limit of the SiO2/Al2O3 ratio is unbounded. ZSM-5 is one such example wherein the SiO2/Al2O3 ratio ranges from at least 5 up to infinity.
Because the catalytic activity of zeolite is affected in part by the size of the channels and pores responsible for selective absorption, micropore size is an important characteristic in these materials. Zeolites that are most widely used catalytic materials in oil refining and petro-chemicals typically have micropores less than 1 nm which limits mass-transfer of molecules into and out of the micropores.
In addition to micropore size affecting the mass-transfer limitation, the morphology of zeolites may also affect absorption and the resulting catalytic activity. Some zeolites with one-dimensional (1D) pore systems have needle and rod-like morphologies where the pore-mouth can be blocked by coke deposition. In addition to the mass-transfer problem, several silicon-aluminate crystals have needle- or rod-like crystal shapes which may also be subject to rapid deactivation due to their morphology.
Needle-like crystals with very high crystal aspect-ratio (length-to-width ratio) are common in certain zeolitic frameworks. Because the one-dimensional channels in these zeolites are parallel to the growth direction of the needle crystals, the catalytic activity of such zeolites can degrade over time due to the resulting long diffusion path.
Zeolite morphology can be affected by controlling the process of crystallization. Zeolite catalysts having efficient mass-transfer properties can be obtained by reducing the crystals size of zeolites to sub-micrometer and even to nanometer range. This mechanism of size reduction is aimed at reducing the diffusion path length of reactant molecules into the active sites in zeolite crystals.
Synthetic zeolites are generally prepared by providing an aqueous solution of the desired oxides and other required components of the crystallization reaction medium and thereafter crystallizing the zeolite under heat and pressure. An ideal crystallization method directed to catalytic zeolites would provide relatively small crystals at higher rates of productivity compared to known hydrothermal crystallization procedures.
Relatively small zeolitic crystals exhibit greater catalytic activity because their relatively small size permits faster diffusion of reactants into the catalytic sites (absorption), as well as faster diffusion of products out of the catalytic sites (desorption). While the rate of nucleation leading to zeolytic crystallization is not greatly influenced by temperature, the crystal growth process itself is more temperature-sensitive so that when conventional hydrothermal processes are utilized in obtaining relatively small crystals it is necessary to reduce the temperature.
Consequently, in order to obtain relatively small crystals, conventional hydrothermal crystallization processes require significantly more time to effect crystallization leading to ideally small zeolyte crystals. Such longer crystallization periods not only reduce productivity, but they also increase the risk that the desired crystalline material will become contaminated with undesirable crystalline material.
One approach to solving this dilemma involves the use of structure-directing or templating agents to bias favorable crystallization. When small-molecule organic compounds are employed for this purpose, they are often referred to as organic structure directing agents (OSDA).
In such an approach, aluminophosphates, for example, may be prepared by hydrothermal crystallization of a reaction mixture containing a reactive source of phosphate, alumina and water and at least one OSDA which can include, for example, an organic amine and a quarternary ammonium salt. Alternatively, silicoaluminophosphates, for example, may be synthesized by hydrothermal crystallization of a reaction mixture containing reactive sources of silica, alumina and phosphate, in the presence of an OSDA, preferably with a compound of an element of Group VA of the Periodic Table, and optionally in the presence of an alkali metal. In both of these methods, hydrothermal crystallization occurs in a reaction vessel inert toward the reaction system by heating until sufficient crystallization is complete—usually for period lasting as long as two weeks. The solid crystalline reaction product is then recovered by any convenient method such as filtration or centrifugation.
Zeolites with the MTT topology are molecular sieves having pores defined by parallel non-intersecting 5-, 6- and 10-membered ring (MR) channels having cross-sectional dimensions of about 4.5 Angstrom by about 5.2 Angstrom. Examples of MTT framework type molecular sieves include ZSM-23, SSZ-32, EU-13, ISI-4 and KZ-I. ZSM-23, which is a MTT zeolite having medium-sized pores composed of one dimensional pores made up of 10-membered rings, is known to be a potential catalyst for dewaxing, skeletal isomerization of paraffin, selective catalytic cracking and related shape-selective reactions.
One variant of ZSM-23 is composed of 10-membered-rings channels having pore diameters of 0.45×0.52 nm. This variant may be useful as a solid-acid catalyst for a number of important refinery processes such as selective cracking and isomerization. However, long-needle crystals of ZSM-23 are known to cause short catalytic lifetime due to blockage of the zeolite pore mouth by coke species. See S. van Donk, J. H. Bitter, K. P. de Jong, Deactivation of solid acid catalysts for butane skeletal isomerization: on the beneficial and harmful effects of carbonaceous deposits, Appl. Catal. A: Gen., 212 (2001) 97-116.
Other limitations of ZSM-23 relate to the inability with the conventional hydrothermal crystallization techniques to adequately control the acidity and the Si/Al ratio of the formed zeolite.
ZSM-23 can be synthesized using different OSDAs such as pyrolidine, isopropyl amine and a range of organic amines and quaternary ammonium templates. See K. Möller, T. Bein, Crystallization and porosity of ZSM-23, Micropor. Mesopor. Mater., 143 (2011) 253-262; C. Baerlocher, W. H. Meier, D. H. Olson, Atlas of Zeolite Frameworks Types (5th ed., 2001) (Elsevier, Amsterdam) 266; G. Kuhl, Verified Syntheses of Zeolitic Materials (2nd ed., 2001) (Elsevier, Amsterdam) 258. Rollmannn and co-workers extended the OSDAs which can be applied in MTT synthesis. See The Atlas of Zeolite Structure Types, www.iza-online.org.
The synthesis of ZSM-23 by conventional hydrothermal synthesis has been reported elsewhere such as U.S. Pat. No. 5,332,566 and U.S. Pat. No. 4,490,342.
The typical synthesis time of ZSM-23 in hydrothermal synthesis is between 66 and 72 hours with a relatively narrow synthesis window. K. Möller, T. Bein, Crystallization and porosity of ZSM-23, Micropor. Mesopor. Mater., 143 (2011) 253-262. Further, the range of acidities (relating to the Si/Al ratio) of the resulting MTT zeolites is considered to be relatively small.
The unique properties of H-ZSM-23 zeolite in particular correspond with its one-dimensional non-interacting 10 member-ring (10-MR) structures having pore channels of 0.46×0.57 nm. A. W. Burton, A Priori Phase Prediction of Zeolites: Case Study of the Structure-Directing Effects in the Synthesis of MTT-Type Zeolites, J. Am. Chem. Soc., 129 (2007) 7627-37. However, blockage of the zeolite pore mouth by the coke and other species of byproducts is considered to account for the relatively rapid deactivation of H-ZSM-23. S. van Donk, J. H. Bitter, K. P. de Jong, Deactivation of solid acid catalysts for butane skeletal isomerization: on the beneficial and harmful effects of carbonaceous deposits, Appl. Catal. A: Gen., 212 (2001) 97-116. Furthermore, common methods to improve porosity such as desilication have resulted in detrimental effects to zeolite morphology, blockage of microporosity, and uncontrolled Al removal. D. S. Kim, J. S. Chang, J. S. Hwang, S. E. Park S E, J. M, Kim, Synthesis of zeolite beta in fluoride media under microwave irradiation, Micropor. Mesopor. Mater., 68 (2004) 77-82.
Efforts to improve mass-transfer in one-dimensional (1D) pore zeolites can be classified into two strategies: (i) scaling-down the crystal size from micrometer to nanometer; and (ii) development of a hierarchical pore system. Möller and Bein reported the preparation of a high crystal-aspect-ratio (length/width) variant of ZSM-23, wherein a crystal-aspect-ratio above 400 was obtained. K. Möller, T. Bein, Crystallization and porosity of ZSM-23, Micropor. Mesopor. Mater., 143 (2011) 253-62. This high aspect-ratio, however, induces fast deactivation and such rod-like morphology can be easily poisoned by cokes tending to block the pore-mouth.
Therefore, development of MTT crystals having shorter crystals with significantly lower crystal-aspect-ratios is one aspect of the present disclosure, because lowering of the crystal-aspect-ratio may reduce diffusion constraints which limit the catalytic effectiveness of ZSM-23. Another aspect of the present disclosure relates to the unfavorable synthesis constraints on the preparation of ZSM-23 using conventional techniques as well as the presence of impurity phases such as cristobalite and dodecasil. See A. W. Burton, A Priori Phase Prediction of Zeolites: Case Study of the Structure-Directing Effects in the Synthesis of MTT-Type Zeolites, J. Am. Chem. Soc., 129 (2007) 7627-37.
Microwave-assisted hydrothermal synthesis (MAHyS) has found many applications in synthetic chemistry to shorten synthesis time, to produce narrow size distribution, and to obtain different morphologies. See Mintova, N. H. Olson, V. Valtchev, T. Bein, Mechanism of Zeolite A nanocrystal growth from colloids at room temperature, Science, 283 (1999) 958-60; D. S. Kim, J. S. Chang, J. S. Hwang, S. E. Park SE, J. M, Kim, Synthesis of zeolite beta in fluoride media under microwave irradiation, Micropor. Mesopor. Mater., 68 (2004) 77-82; O. Muraza, E. V. Rebrov, J. Chen, M. Putkonen, L. Niinistö L, M. H. J. M, de Croon, J. C. Schouten, Microwave-assisted hydrothermal synthesis of zeolite Beta coatings on ALD-modified borosilicate glass for application in microstructured reactors, Chem. Eng. J., 135 (2008) S117-S120. A range of nanozeolites have been reported using MAHyS (e.g., MFI, BEA, LTL, and LTA).
In some instances, the use of MAHyS appears to reduce the time required for zeolite crystallization. For instance, Li and co-workers reported rapid fabrication of MOR zeolites by MAHyS where the synthesis time was reduce to 6 hours at 190° C. G. Li, H. M. Hou, R. S. Lin, Rapid synthesis of mordenite crystals by microwave heating, Solid State Sci., 13 (2011) 662-64. The use of MAHyS in the presence of OSDAs has also been reported for preparing a number of zeolites. See, e.g., U.S. Pat. No. 4,778,666.
Although MAHyS can be used with OSDAs to affect greater control of the crystallization of certain zeolites. The existing methodology is insufficiently rapid and reliable for producing MTT zeolite crystals such as ZSM-23 possessing the characteristics of relatively small pore size and crystal-aspect-ratio. Improved morphologies applicable to catalysts having significantly longer catalytic lifetimes than previously known ZSM-23 zeolites are therefore needed.
The present disclosure has been made to address the above-described problems.
One objective of this disclosure is to provide a method for controlled nucleation and growth of MTT zeolites by using MAHyS, to allow a synthesis of MTT zeolites such as ZSM-23 which is rapid and produces MTT zeolites having relatively small crystal sizes with low crystal-aspect-ratios and with improved morphologies that are less susceptible to impurities and catalytic deactivation. A further objective of the present disclosure is to provide MTT zeolites having improved catalytic properties by optimization of synthesis parameters and by employing techniques such as structure-directed synthesis, seed-assisted synthesis and alkaline post-treatment.
Additional objects, advantages and other features of the present disclosure will be set forth in part in the description that follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present disclosure. The advantages of the present disclosure may be realized and obtained as particularly pointed out in the appended claims. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the present disclosure. In this regard, the description herein is to be understood as illustrative in nature, and not as restrictive.